1. Field of the Invention
The present invention relates to a submount substrate for mounting a light emitting device in which a Zener diode used as a voltage regulator device is integrated, and a method of fabricating the same. More particularly, the present invention relates to a submount substrate for mounting a light emitting device in which a PN Zener diode or a Zener diode having a bi-directional threshold voltage characteristic is integrated using a silicon bulk micromachining process, and a method of fabricating the same.
2. Description of the Related Art
In general, a light emitting device such as a light emitting diode or a laser diode using a group III to V compound semiconductor material of a direct transition type semiconductor can generate a variety of colored lights including green, blue, ultraviolet and the like as a result of due to the advancement in the field of thin film growth technologies and semiconductor materials.
Furthermore, a white light with good efficiency can also be embodied by using a fluorescence material or combining a variety of colors.
The technological advancement thus described enables the light emitting device to encompass a wide range of applications such as a transmission module for optical-communication system, a light emitting diode (LED) backlight capable of substituting for the cold cathode fluorescence lamp (CCFL) constituting the backlight of a liquid crystal display (LCD) device, a white LED system capable of substituting for a fluorescent lamp or incandescent lamp, and a traffic light, in addition to a display device.
FIG. 1 is a cross-sectional view of a general LED device. In order to fabricate this LED device, a buffer layer (102), an n-contact layer (103), an n-cladding layer (not shown), an active layer (104), a p-cladding layer (not shown), and a p-contact layer (105) are sequentially deposited on the top of a substrate (101) made of sapphire, n-GaAs, GaN or the like, using a chemical vapor deposition process.
Thereafter, a mesa etching is carried out to expose the n-contact layer (103), through a photolithographic etching process and a dry/wet etching process.
Next, on the top of the p-contact layer (105) is formed a current diffusion layer (106), which is formed of a transparent electrode with good light transmission. For an electrical connection with an external circuit, a p-electrode (107-p) and an n-electrode (107-n) are formed on the current diffusion layer (106) and the n-contact layer (103), respectively, thereby forming a LED device (100).
In other words, when a voltage from an external circuit is applied between the p-electrode (107-p) and the n-electrode (107-n) in the light-emitting device, positive holes and electrons are injected into the p-electrode (107-p) and the n-electrode (107-n). While the positive holes and the electrons are recombined in the active layer (104), extra energy is converted into light, which in turn is emitted to the outside through the current diffusion layer and the substrate.
In the meantime, if static electricity or surge voltage is produced in this type of light emitting device, an excessive electric charge flows into the semiconductor layers and finally causes failure of the light emitting device.
This problem becomes worse in a case where the devices are fabricated on the top of a dielectric substrate. When the surge voltage is produced in the device, an applied voltage may rise up to a few thousands volts. Thus, when a light emitting device has a low withstand voltage (allowable voltage), an additional protective device should be installed.
As a protective device, for example, a plurality of general diodes are connected in series such that the diodes can be turned on at a voltage higher than the driving voltage of the light emitting device.
Therefore, as shown in FIGS. 2a and 2b, a PN or PNP (NPN) Zener diode (200. 300), which is used as a constant voltage device, is connected to a light emitting device (100) in such a manner that their opposite electrodes are connected to each other. Thus, the voltage applied to the light emitting device is restricted to Vz (Zener voltage) of the Zener diode.
If the reverse voltage of the Zener diode is equal to or greater than Vz, the reverse current (a current flowing from the n-electrode towards the p-electrode) increases and the terminal voltage between both ends of the Zener diode remains almost constant, i.e. Vz.
In this way, a Zener diode is not only used as a protective device but also widely used as a voltage regulator device for maintaining a load voltage to be constant against variation in an input voltage or in a load.
Such a Zener diode as a protective device is fabricated separately from the device, such as a light emitting device, to be protected and is then electrically connected thereto in parallel. Alternatively, the light emitting device and the Zener diode may be connected on a silicon submount substrate by means of a flip chip bonding.
FIGS. 3a to 3g are cross-sectional views explaining a process of fabricating a conventional PN Zener diode. FIGS. 3a and 3b show a process of forming masks.
First, a Zener diode is a device that utilizes the tunneling effect in quantum mechanics, and thus, a substrate (201) with low resistance must be used. Since Zener voltage (Vz) of the device is determined by the electrical resistivity of the substrate and the concentration of diffused impurities thereof, a substrate with an appropriate concentration of impurities contained therein must be used (see FIG. 3a).
Further, in order to selectively diffuse impurities into the substrate, a diffusion mask (202) (a silicon oxide film generally employed) is deposited on the top and bottom surfaces of the substrate. Then, the diffusion mask deposited on the top surface of the substrate is selectively etched and then patterned (see FIG. 3b).
FIG. 3c shows a diffusion step. After patterning the diffusion mask, a diffusion process is carried out. Impurities different from those in the substrate are injected into the substrate through a portion (portion “B” of FIG. 3C) where the diffusion mask is etched. The impurity injection process can utilize a diffusion process and an ion injection process using a furnace.
Here, impurities are not injected into the substrate through a portion “A” where the diffusion mask remains, because the impurities are covered by the diffusion mask.
Thereafter, the diffusion mask is removed (see FIG. 3d), and a protective film (203) is deposited on the top surface of the substrate (201). Then, the diffusion area D of the substrate (201) is exposed (see FIG. 3e).
Finally, as shown in FIGS. 3f and 3g, on the top surface of the exposed diffusion area (D) of the substrate (201) is formed an electrode (204-f), and on the bottom surface of the substrate (201) is formed an electrode (204-b). Consequently, a Zener diode (200) is fabricated.
As described above, the Zener diode according to the prior arts requires several processes such as a diffusion mask deposition process, a diffusion mask photo process and a diffusion mask etching process. Accordingly, there is a problem in that manufacturing costs can not be reduced due to increased the number of manufacturing processes.